PML side walls in HFSS

In the tutorial that comes with the software, it explains how to set up a PML on the top of an air box. An impedance boundary with 377*cos(theta_scan) Ohms is supposed to be set on the top face of the PML.

Could anybody explain how I can set up PMLs on the side walls of an air box? To be exact, my question is like what impedance I should use on the outside of the PML?

Thanks in advance!

Perhaps, I can use 377*sin(theta_scan)?